The Enhanced ER Model Database Management System 1

The Enhanced E-R Model Database Management System 1 (ITED 123 A) ITE Department

Objectives • Define terms • Understand use of supertype/subtype relationships • Understand use of specialization and generalization techniques • Specify completeness and disjointness constraints • Develop supertype/subtype hierarchies for realistic business situations • Develop entity clusters ITE Department

Supertypes and Subtypes • Enhanced ER model: extends original ER model with new modeling constructs • Subtype: A subgrouping of the entities in an entity type that has attributes distinct from those in other subgroupings • Supertype: A generic entity type that has a relationship with one or more subtypes • Attribute Inheritance: – Subtype entities inherit values of all attributes of the supertype – An instance of a subtype is also an instance of the supertype ITE Department

Figure 3 -1 Basic notation for supertype/subtype notation a) EER notation ITE Department 4

Figure 3 -1 Basic notation for supertype/subtype notation (cont. ) b) Microsoft Visio Notation Different modeling tools may have different notation for the same modeling constructs. ITE Department 5

Figure 3 -2 Employee supertype with three subtypes All employee subtypes will have employee number, name, address, and date hired Each employee subtype will also have its own attributes ITE Department 6

Relationships and Subtypes • Relationships at the supertype level indicate that all subtypes will participate in the relationship • The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level ITE Department

Figure 3 -3 Supertype/subtype relationships in a hospital ITE Department 8

Generalization and Specialization • Generalization: The process of defining a more general entity type from a set of more specialized entity types. BOTTOM-UP • Specialization: The process of defining one or more subtypes of the supertype and forming supertype/subtype relationships. TOP-DOWN ITE Department

Figure 3 -4 Example of generalization a) Three entity types: CAR, TRUCK, and MOTORCYCLE All these types of vehicles have common attributes ITE Department 10

Figure 3 -4 Example of generalization (cont. ) b) Generalization to VEHICLE supertype So we put the shared attributes in a supertype Note: no subtype for motorcycle, since it has no unique attributes ITE Department 11

Figure 3 -5 Example of specialization a) Entity type PART Only applies to manufactured parts Applies only to purchased parts ITE Department 12

Figure 3 -5 Example of specialization (cont. ) b) Specialization to MANUFACTURED PART and PURCHASED PART Created 2 subtypes Note: multivalued composite attribute was replaced by an associative entity relationship to another entity ITE Department 13

Constraints in Supertype/Subtype Relationships • Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype – Total Specialization Rule: Yes (double line) – Partial Specialization Rule: No (single line) ITE Department

Figure 3 -6 Examples of completeness constraints a) Total specialization rule ITE Department 15

Figure 3 -6 Examples of completeness constraints (cont. ) b) Partial specialization rule ITE Department 16

Constraints in Supertype/Subtype Relationships • Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes – Disjoint Rule: An instance of the supertype can be only ONE of the subtypes – Overlap Rule: An instance of the supertype could be more than one of the subtypes ITE Department 17

Figure 3 -7 Examples of disjointness constraints a) Disjoint rule ITE Department 18

Figure 3 -7 Examples of disjointness constraints (cont. ) b) Overlap rule ITE Department 19

Constraints in Supertype/Subtype Relationships • Subtype Discriminator: An attribute of the supertype whose values determine the target subtype(s) – Disjoint – a simple attribute with alternative values to indicate the possible subtypes – Overlapping – a composite attribute whose subparts pertain to different subtypes. Each subpart contains a Boolean value to indicate whether or not the instance belongs to the associated subtype ITE Department 20

Figure 3 -8 Introducing a subtype discriminator (disjoint rule) ITE Department 21

Figure 3 -9 Subtype discriminator (overlap rule) ITE Department 22

Figure 3 -10 Example of supertype/subtype hierarchy ITE Department 23

Entity Clusters • EER diagrams are difficult to read when there are too many entities and relationships. • Solution: Group entities and relationships into entity clusters. • Entity cluster: Set of one or more entity types and associated relationships grouped into a single abstract entity type ITE Department

Figure 3 -13 a Possible entity clusters for Pine Valley Furniture in Microsoft Visio Related groups of entities could become clusters ITE Department

Figure 3 -13 b EER diagram of PVF entity clusters More readable, isn’t it? ITE Department 26

Figure 3 -14 Manufacturing entity cluster Detail for a single cluster ITE Department 27

Summary In this lesson, you should have learned the following: – Use of supertype/subtype relationships – Use of specialization and generalization techniques – Specify completeness and disjointness constraints – Develop supertype/subtype hierarchies for realistic business situations – Develop entity clusters ITE Department

Reference • Hoffer, J. , Ramesh, V. , Topi, H. (2013). Modern Database Management 11 th Edition, Prentice Hall. ITE Department